Lens systems of wide relative aperture are generally capable of forming bright images at low light levels. This capability is especially advantageous in infrared imaging applications in which the image-forming radiation consists of thermal emissions from the objects that are being imaged. However, lens systems of wide relative aperture have hitherto been capable of forming high-resolution images only over small fields of view (i.e., less than about 30.degree.).
In a distortion-free imaging system, the image size is proportional to the tangent of the field angle. Consequently, if an imaging lens system having an extremely wide field of view is to be perfectly distortion free, the size of the image plane is correspondingly extremely large. However, in many applications requiring a wide-field lens system, it is beneficial to permit a certain amount of image distortion in order to accommodate an image plane of reasonable size. In practice, for many wide-field imaging applications, a condition in which image size is substantially proportional to the field angle itself (rather than to the tangent of the field angle) is deemed to provide acceptably small image distortion. The condition in which image size is substantially proportional to the field angle for a lens system having a wide field of view is called the f-.theta. condition.
Until the present invention, design forms had not been developed for infrared imaging systems that substantially meet the f-.theta. condition simultaneously for wide fields of view and for wide relative apertures.
A need has been experienced in the prior art for infrared lens systems having wide fields of view as well as wide relative apertures, which substantially meet the f-.theta. condition and have a limiting aperture stop in converging image space. For a lens system used in an infrared detection device, location of the limiting aperture stop in converging image space would allow the entire region of the lens system from the physical aperture stop to the image plane to be shielded and cooled (e.g., to cryogenic temperatures) so as to limit the amount of thermal radiation from the surrounding environment that can reach the detector. However, until the present invention, design forms had not been developed for infrared lens systems having wide fields of view (e.g., greater than one steradian) and wide relative apertures (e.g., greater than one-half focal length) for which the limiting aperture stop is located in converging image space.